Electro-photographic (EP) printing devices form images on print media by placing a uniform electrostatic charge on a photoconductor and then selectively discharging the photoconductor in correspondence with the images. The selective discharging forms a latent electrostatic image on the photoconductor. Colorant is then developed onto the latent image of the photoconductor, and the colorant is ultimately transferred to the media to form the image on the media. In dry EP (DEP) printing devices, toner is used as the colorant, and it is received by the media as the media passes below the photoconductor. The toner is then fixed in place as it passes through heated pressure rollers. In liquid EP (LEP) printing devices, ink is used as the colorant instead of toner. In LEP devices, an ink image developed on the photoconductor is offset to an image transfer element, where it is heated until the solvent evaporates and the resinous colorants melt. This image layer is then transferred to the surface of the print media being supported on a rotating impression drum.
Achieving high print quality (PQ) with an electrophotographic printing device depends in part on keeping the photoconductor clean, so that it has a high surface resistivity that can maintain the electrostatic latent image. However, during the normal printing process, the photoconductive surface accumulates contamination and becomes oxidized. The photoconductive surface can also absorb moisture. The contaminants, oxidation, and moisture, can create lateral conductivity across the surface, resulting in poor PQ, blurriness of edges, and elimination of small elements such as dots and lines.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.